Generally, a bearing is arranged in a rotary shaft portion installed in various types of drive force transmission mechanisms or a rotary shaft (output shaft) portion of a drive motor. Typically, as such a bearing, a so-called ball bearing is often used that houses therein a plurality of rolling members (rolling elements) between an inner ring and an outer ring along a circumferential direction thereof, and this type of bearing is used so that rotation performance of a rotary shaft can be improved.
As disclosed in, for example, Japanese Patent Application Publication No. 2015-10673, the above-described type of bearing is known to have a configuration in which a magnetic fluid seal mechanism is incorporated thereinto (a magnetic fluid sealed bearing) in order to prevent foreign substances such as dust and water from entering inside. The magnetic fluid seal mechanism incorporated into this bearing is configured as follows. That is, a ring-shaped magnet is mounted on an outer peripheral surface of the inner ring or an inner peripheral surface of the outer ring as a fixed side to form a magnetic circuit, and a magnetic fluid is retained in a gap on an opposite side (a gap on the inner peripheral surface of the outer ring or a gap on the outer peripheral surface of the inner ring) so as to prevent foreign substances from entering the rolling members.
Furthermore, there is also known a configuration in which the magnetic seal mechanism is not incorporated into the bearing itself but is arranged so as to protect the bearing arranged between a rotary shaft and a housing portion such as a frame. For example, Japanese Patent Application Publication No. 2014-161300 (the '300 Publication) discloses a drive motor having a configuration in which, with respect to a bearing arranged between a frame (a housing) and a drive shaft of the drive motor for an electric reel, a flange portion is formed integrally with the frame, and a magnetic seal mechanism that retains a magnetic fluid between itself and the drive shaft is arranged in the flange portion so as to prevent foreign substances from entering the bearing.
A magnetic fluid sealed bearing as described above is configured so that a magnetic fluid is retained by a magnetic circuit formed by a magnet, and, therefore, care should be taken when handling it and incorporating it into a rotary shaft portion. For example, when the magnetic fluid sealed bearing is placed in a portion having magnetism, such as a metallic work table, there is a possibility that the magnetic circuit is affected, causing the magnetic fluid to flow out. To avoid this, it is also conceivable to inject the magnetic fluid after the magnetic fluid sealed bearing has been incorporated into the rotary shaft portion. This, however, deteriorates incorporation workability.
Furthermore, when, as disclosed in the '300 Publication, the magnetic seal mechanism is arranged in the frame of the drive motor adjacently to the bearing of the drive motor, a magnetic circuit formed by a magnet in the magnetic seal mechanism is affected by the frame made of a magnetic material and thus becomes no longer stable. Thus, in order to stably retain a magnetic fluid in a predetermined gap portion, a magnetic circuit should be formed to be stable.